This invention relates to an aqueous composition suitable for use in coating leather and a method for coating leather with an aqueous coating composition to provide coated leather having good embossability and wet-flex endurance. More particularly this invention relates to an aqueous composition including an emulsion polymer including from 0.4% to 10% by weight of copolymerized acetoacetate or acetoacetamide monomer based on the weight of the polymer and having a glass transition temperature (Tg) from xe2x88x9220xc2x0 C. to 10xc2x0 C. More particularly, in a second embodiment, this invention relates to an aqueous composition including an emulsion polymer including from 0.1% to 6% by weight of copolymerized acetoacetate or acetoacetamide monomer based on the weight of the polymer and from 2% to 15% by weight of copolymerized carboxylic acid monomer based on the weight of the polymer and having a glass transition temperature (Tg) from xe2x88x9220xc2x0 C. to 10xc2x0 C., the polymer having been contacted with a transition metal oxide, hydroxide, or carbonate at a pH of less than 9 in an amount greater than 0.20 equivalent of transition metal per equivalent of the copolymerized carboxylic acid monomer in the polymer.
The present invention serves to provide an aqueous composition suitable for use in coating leather, particularly a basecoating, that is aesthetically pleasing and protective and a method for coating leather. The protective properties of the leather coating may be measured by the flexing endurance of the coated leather, particularly under wet conditions. The coating may be subsequently embossed for decorative purposes with a desired imprint in a heated press. The softness of the final coated leather, the ability of the coating to be embossed easily without clinging to the heated press and the retention of the desired imprint are measured aesthetic properties of the coated leather.
U.S. Pat. No. 5,202,375 discloses a water-resistant polymeric emulsion and coatings containing the emulsion applied to particle board and plywood. The polymeric emulsion polymer contains from about 1 to about 5 weight percent olefinic carboxylic acid monomer and from about 0.5 to about 5 weight percent crosslinking monomer such as, for example, acetoacetoxyethyl methacrylate; and has a Tg of about xe2x88x9250xc2x0 C. to about 50xc2x0 C., preferably from about 0xc2x0 C. to about 20xc2x0 C.
The problem faced by the inventors is the provision of an aqueous composition suitable for use in coating leather that is aesthetically pleasing and protective and a method for coating leather which yields a dried coating on leather that has good embossability and wet-flex resistance.
In a first aspect of the present invention there is provided an aqueous composition suitable for use in coating leather including an emulsion polymer including from 0.4% to 10% by weight of copolymerized acetoacetate or acetoacetamide monomer based on the weight of the polymer and having a glass transition temperature (Tg) from xe2x88x9220xc2x0 C. to 10xc2x0 C.
In a second aspect of the present invention there is provided an aqueous composition suitable for use in coating leather including an aqueous emulsion polymer including from 0.1% to 6% by weight of copolymerized acetoacetate or acetoacetamide monomer based on the weight of the polymer and from 2% to 15% by weight of copolymerized carboxylic acid monomer based on the weight of the polymer and having a glass transition temperature (Tg) from xe2x88x9240xc2x0 C. to 0xc2x0 C.; the polymer having been contacted with a transition metal oxide, hydroxide, or carbonate at a pH of less than 9 in an amount greater than 0.20 equivalent of transition metal per equivalent of copolymerized carboxylic acid monomer in the polymer.
In a third aspect of the present invention there is provided a method for coating leather including (a) forming an aqueous composition including an emulsion polymer including from 0.4% to 10% by weight of copolymerized acetoacetate or acetoacetamide monomer based on the weight of the polymer and having a glass transition temperature (Tg) from xe2x88x9220xc2x0 C. to 10xc2x0 C.; (b) coating leather with the aqueous composition; and (c) drying, or allowing to dry, the aqueous composition.
In a fourth aspect of the invention there is provided a method for coating leather including (a) forming an aqueous composition including an emulsion polymer including from 0.1% to 6% by weight of copolymerized acetoacetate or acetoacetamide monomer based on the weight of the polymer and from 2% to 15% by weight of copolymerized carboxylic acid monomer based on the weight of the polymer and having a glass transition temperature (Tg) from xe2x88x9240xc2x0 C. to 0xc2x0 C.; (b) contacting the polymer with a transition metal oxide, hydroxide, or carbonate at a pH of less than 9 in an amount greater than 0.20 equivalent of transition metal per equivalent of the copolymerized carboxylic acid monomer in the polymer; (c) coating leather with the aqueous composition; and (d) drying, or allowing to dry, the aqueous composition.
This invention relates to a composition suitable for use in coating leather and a method for coating leather with an aqueous composition including an emulsion polymer including at least one copolymerized ethylenically unsaturated monomer and from 0.4% to 10% by weight, preferably from 0.4% to 4% by weight, of copolymerized acetoacetate or acetoacetamide monomer and has a glass transition temperature (Tg) from xe2x88x9220xc2x0 C. to 10xc2x0 C. The emulsion polymer contains at least one copolymerized ethylenically unsaturated monomer exclusive of copolymerized acetoacetate or acetoacetamide monomer such as, for example, a (meth)acrylic ester monomer including methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, lauryl acrylate, methyl methacrylate, butyl methacrylate, isodecyl methacrylate, lauryl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, aminoalkyl (meth)acrylates; styrene or substituted styrenes; butadiene; vinyl acetate or other vinyl esters; vinyl monomers such as vinyl chloride, vinylidene chloride, N-vinyl pyrollidone; and acrylonitrile or methacrylonitrile. The use of the term xe2x80x9c(meth)xe2x80x9d followed by another term such as acrylate or acrylamide, as used throughout the disclosure, refers to both acrylates or acrylamides and methacrylates and methacrylamides, respectively. Preferred is the absence of copolymerized monomers containing functional group(s) capable of chemical reaction with acetoacetate or acetamide groups such as, for example aldehyde and amine groups. Preferred is a polymer including 25-65% by weight of copolymerized ethyl acrylate.
The emulsion polymer contains from 0.4% to 10% by weight of copolymerized acetoacetate or acetoacetamide monomer, i.e., an ethylenically unsaturated monomer containing an acetoacetate or acetoacetamide grouping, such as, for example, vinyl acetoacetate, acetoacetoxyethyl (meth)acrylate, acetoacetoxypropyl (meth)acrylate, allyl acetoacetate, acetoacetoxybutyl (meth)acrylate, 2,3-di(acetocacetoxy)propyl (meth)acrylate, vinyl acetoacetamide, and acetoacetoxyethyl (meth)acrylamide.
The emulsion polymer may also contain from 0% to 10% of a copolymerized monoethylenically-unsaturated carboxylic acid monomer, based on the weight of the polymer, such as, for example, acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, monomethyl itaconate, monomethyl fumarate, monobutyl fumarate, and maleic anhydride. Preferred carboxylic acid monomers are acrylic acid, methacrylic acid, and itaconic acid.
The emulsion polymer used in this invention may also contain from 0% to 5% copolymerized multi-ethylenically unsaturated monomers such as, for example, allyl methacrylate, diallyl phthalate, 1,4-butylene glycol dimethacrylate, 1,2-ethylene glycol dimethacrylate, 1,6-hexanediol diacrylate, and divinyl benzene.
The glass transition temperature (xe2x80x9cTgxe2x80x9d) of the emulsion polymer is from xe2x88x9220xc2x0 C. to 10xc2x0 C., as measured by differential scanning calorimetry (DSC) taking the mid-point in the heat flow versus temperature transition as the Tg value.
The polymerization techniques used to prepare aqueous emulsion-polymers are well known in the art. In the emulsion polymerization process conventional surfactants may be used such as, for example, anionic and/or nonionic emulsifiers such as, for example, alkali metal or ammonium alkyl sulfates, alkyl sulfonic acids, fatty acids, and oxyethylated alkyl phenols. The amount of surfactant used is usually 0.1% to 6% by weight, based on the weight of monomer. Either thermal or redox initiation processes may be used. The monomer mixture may be added neat or as an emulsion in water. The monomer mixture may be added in one or more additions or continuously over the reaction period allotted. Addition the monomer in a single portion at the beginning of the reaction is preferred. Conventional free radical initiators may be used such as, for example, hydrogen peroxide, t-butyl hydroperoxide, ammonium and/or alkali persulfates, typically at a level of 0.01% to 3.0% by weight, based on the weight of total monomer. Redox systems using the same initiators coupled with a suitable reductant such as, for example, sodium sulfoxylate formaldehyde, sodium hydrosulfite, isoascorbic acid, and sodium bisulfite may be used at similar levels. Chain transfer agents such as mercaptans may be used to lower the molecular weight of the formed polymer of one or more of the stage polymers; the use of no chain transfer agent is preferred.
The average particle diameter of the emulsion-polymerized polymer particles is preferred to be from 30 nanometers to 500 nanometers.
In an alternative embodiment the emulsion polymer may be prepared in a multistage polymerization process in which two or more polymeric stages varying in composition are prepared in sequential fashion. The polymerization techniques used to prepare such aqueous multi-stage emulsion-polymers are well known in the art such as, for example, U.S. Pat. Nos. 4,325,856; 4,654,397; and 4,814,373. A preferred two-stage polymer has at least 80% by weight based on the weight of the polymer of a first polymer containing from 0.1% to 10% by weight of copolymerized acetoacetate or acetoacetamide monomer and having a Tg from xe2x88x9240xc2x0 C. to 0xc2x0 C., and 2% to 20% by weight based on the weight of the polymer of a second polymer containing from 0% to 10% by weight of copolymerized acetoacetate or acetoacetamide monomer and having a Tg at least 10xc2x0 C. higher than the Tg of the first polymer. Such a process usually results in the formation of at least two mutually incompatible polymer compositions, thereby resulting in the formation of at least two phases. The mutual incompatibility of two polymer compositions and the resultant multiphase structure of the polymer particles may be determined in various ways known in the art. The use of scanning electron microscopy using staining techniques to emphasize the difference between the appearance of the phases, for example, is such a technique.
This invention also relates to an aqueous composition suitable for use in coating leather of the second aspect of the invention including an aqueous emulsion polymer including from 0.1% to 6% by weight, preferably from 0.1% to 4%, more preferably from 0.4% to 1% by weight, of copolymerized acetoacetate or acetoacetamide monomer based on the weight of the polymer, and from 2% to 15% by weight, preferably from 4% to 5% by weight, of copolymerized carboxylic acid monomer based on the weight of the polymer and having a glass transition temperature (Tg) from xe2x88x9240xc2x0 C. to 0xc2x0 C., preferably from xe2x88x9240xc2x0 C. to xe2x88x928xc2x0 C.; the polymer having been contacted with a transition metal oxide, hydroxide, or carbonate at a pH of less than 9 in an amount greater than 0.20 equivalent, preferably greater than 0.50 equivalent, of transition metal per equivalent of copolymerized carboxylic acid monomer in the polymer. The components, but not necessarily the quantities, ranges, or values of the elements, of the aqueous emulsion polymer of this second aspect such as the copolymerized monomer, the copolymerized acetoacetate or acetamide monomer, the copolymerized carboxylic acid, and the Tg are the same as defined hereinabove for the first aspect, as is the method of preparation and adjuvants used therein.
The emulsion polymer of the second aspect is contacted with a transition metal oxide, hydroxide, or carbonate at pH less than pH=9, preferably at pH =3-6, in an amount greater than 0.20 equivalent, preferably greater than 0.50 equivalent, of transition metal per equivalent of copolymerized carboxylic acid monomer in the emulsion polymer according to the process disclosed in U.S. Pat. No. 5,221,284. The oxides, hydroxides, and carbonates of zinc, aluminum, tin, tungsten, and zirconium are preferred for low cost, low toxicity, and low color in the dried coating. Zinc oxide is more preferred. The transition metal oxide, hydroxide, or carbonate may be added slurried in water, optionally with an added dispersant such as, for example a low molecular weight polymer or copolymer of (meth)acrylic acid. The transition metal oxide, hydroxide, or carbonate may be added during the polymerization process or after the polymerization has been completed. Alternatively, the transition metal may be added in a soluble form such as a solution of zinc ammonium carbonate added after the formation of the emulsion polymer and neutralization of the emulsion polymer to a pH greater than 8.
The aqueous composition is prepared by techniques which are well known in the coatings art. First, optionally, at least one pigment is well dispersed in an aqueous medium under high shear such as is afforded by a COWLES (R) mixer or, in the alternative, at least one predispersed colorant is used. Then the emulsion polymer is added under low shear stirring along with other coatings adjuvants as desired. The aqueous coating composition may contain, in addition to the optional pigment(s) and the emulsion polymer, conventional coatings adjuvants such as, for example, emulsifiers, coalescing agents, buffers, neutralizers, thickeners, humectants, wetting agents, biocides, plasticizers, antifoaming agents, colorants, waxes, and anti-oxidants. Preferred is an aqueous composition free from organic compounds or polymers containing functional group(s) capable of chemical reaction with acetoacetate or acetamide groups such as, for example aldehydes and amines.
The solids content of the aqueous coating composition may be from about 10% to about 50% by volume. The viscosity of the aqueous composition may be from 0.05 to 10 Pa.s (50 cps to 10,000 cps), as measured using a Brookfield viscometer; the viscosities appropriate for different application methods vary considerably.
The aqueous coating composition may be applied to leather such as, for example, mineral tanned or vegetable tanned leather including full-grain leather, buffed or corrected-grain leather, and split leather with or without a prior treatment with an impregnating resin mixture and with or without the application of subsequent coatings using conventional coatings application methods such as, for example, curtain coater and spraying methods such as, for example, air-atomized spray, air-assisted spray, airless spray, high volume low pressure spray, and air-assisted airless spray.
The aqueous coating composition coated on leather is typically dried, or allowed to dry, at a temperature from 20xc2x0 C. to 75xc2x0 C.
Flexing Endurance of Coated Leather
The flexing endurance under wet or dry conditions is based on the IUF 20 method of International Union of Leather Chemists Association using a Bally Flexometer (Bally SchuhFabriken AG, Schoenenwerd, Switzerland). The dry or wet leather specimens (65 mm. by 40 mm.) were flexed and examined for the extent of cracking after the reported number of flexes.
Embossing Tests
The coated leather was embossed in a Turner-type press at 85-95xc2x0 C. and 70.3 kg/cm2 (1000 psi) for 5-7 seconds and the ease of removability from the hot embossing press (xe2x80x9cplate clingxe2x80x9d)was evaluated using a haircell plate. The print quality was evaluated by examining the embossed haircell pattern for distinctness and sharpness. Cutting, which is the ability to receive deep prints without tearing of the coating, was evaluated by examining the embossed pattern made using a Llama plate for cuts in the film with a 10-power microscope.
Abbreviations
EA=ethyl acrylate
BA=butyl acrylate
AN=acrylonitrile
AA=acrylic acid
EHA=2-ethylhexyl acrylate
AAEM=2-acetoacetylethyl methacrylate
BMA=butyl methacrylate
ACRYSOL and HYDROLAC are trademarks of Rohm and Haas Company. EUDERM, BAYDERM, and EUKANOL are trademarks of Bayer AG.
The following examples are presented to illustrate the invention and the results obtained by the test procedures.